There are numerous processes that result in the undesirable entrainment of particulates or other contaminants within a fluid stream that are preferably removed for various purposes. For example, contaminants are removed from a gas or liquid streams in order to meet environmental regulations, to recover the entrained materials for sale or reuse, or to upgrade the gas or liquid stream for its intended purpose. The removal of contaminant materials from a fluid stream is also required in numerous petroleum, chemical, and biofuel applications, including the following: solids are removed in a biomass gasification plant from the exhaust gas stream; fly ash is removed from the exhaust gas stream of a coal-fired power plant in order to meet applicable pollution regulations; sulfur dioxide gas is also removed from the same exhaust stream from the coal-fired power plant; and any process in which a gas or vapor reacts with a solid, such as in a fluidized bed reaction vessel of a chemical process or in a catalytic process at a petroleum refinery.
One method for contaminant removal is the “moving granular bed filter” or “MGBF.” MGBF employs a moving bed of filter media to remove contaminants from fluid streams. The major development of the moving bed filter was to allow the bed material to move continuously down through the filter vessel and be carried back up to the top and through a cleansing zone before reuse. Such filters provide better cleansing of the bed and the filter is truly continuous, never needing to be shut down for backwash.
An exemplary moving bed filter is illustrated in FIG. 1 below. Fluid e.g., raw water, is fed in (1) and (2) evenly distributes the flow into the bed. The water flows through the sand bed and exits overflow weir (5). The sand bed moves continuously down, being sucked from the bottom by airlift pump (6), carried upward, and washed in washer (7). Also shown are vessel (3), air diffuser (9), and sludge weir (10).
Moving or fluid bed catalysts are also known, and offer the same advantages of continuous catalyst regeneration without the need to shut down the system. Catalytic filters are also known, but these are typically solid filters. However, heretofore, no-one has ever combined both processes into a single “moving bed catalytic filter.”
One application where moving granular bed filters have been successfully employed at high temperatures and pressures is in biomass gasification plants that produce synthesis gas and fast pyrolysis plants that produce pyrolysis oils. “Pyrolysis oil,” also known as “bio-oil,” is an intermediate fuel under investigation as substitute for petroleum. It is extracted by biomass to liquid technology of destructive distillation from dried biomass in a reactor at temperature of about 500° C. with subsequent cooling. Generally speaking, fast pyrolysis has three main products which include bio-oil, char and various gases that are not condensable except at extreme conditions (H2, CO, CO2, CH4). The char and non-condensable gases may be recovered and burned to supply energy to the system, but the condensable gases are rapidly cooled to form condensate droplets that can then be separated from the non-condensable gases due to the substantial difference in density. The composition of two exemplary bio-fuels produced by fast pyrolysis is shown below:
Source: Piskorz, J., et al. In Pyrolysis Oilsfrom Biomass, Soltes, E. J., Milne, T. A.,WhiteEds., ACS Symposium Series 376, 1988.SprucePoplarMoisture content, wt %7.03.3Particle size, μm (max)1000590Temperature500497Apparent residence time0.650.48Product Yields, wt %, m.f.Water11.612.2Gas7.810.8Bio-char12.27.7Bio-oil66.565.7Bio-oil composition, wt %, m.f.Saccharides3.32.4Anhydrosugars6.56.8Aldehydes10.114.0Furans0.35—Ketones1.241.4Alcohols2.01.2Carboxylic acids11.08.5Water-Soluble - Total Above34.534.3Pyrolytic Lignin20.616.2Unaccounted fraction11.415.2
While the biomass begins with about 2% to about 15% moisture, the oil can have a moisture content ranging from about 10% to about 30% or higher. Further, the high oxygen content makes the oil polar and acidic due to the presence of organic acids and the acidity makes the oil corrosive and difficult to store and transport. The pyrolysis oil also has a tendency to polymerize when heating to relatively low temperatures, and the oil is unstable, reacting with air and degassing.
Thus, pyrolysis oils must be upgraded for use and are generally treated to both stabilize the oil and to reduce the oxygen content. One option for upgrading pyrolysis oil is hydrotreating at mild temperatures (270-280° C.). A low temperature hydrotreatment enables stabilization through reactions like olefin, carbonyl and carboxylic groups reduction. Further hydrotreatment at higher temperatures aims at hydrodeoxygenation of phenols and hydrocracking of larger molecules.
Catalytic hydrotreating of bio-oil uses hydrogen in combination with heterogeneous catalysts, such as CoMo/Al2O3, NiMo/Al2O3, or uses catalysts such as HZSM-5 zeolite. One major drawback of using catalysts like CoMo/Al2O3 is the high hydrogen consumption and high pressures needed, which have a strong negative impact on the process economics. HZSM-5 can be used without hydrogen and at atmospheric pressure. However, the catalyst and bio-oil are rapidly degraded at the high temperatures used and yields are poor.
Thus, current pyrolysis oils are difficult to upgrade to transportation fuels because catalyst life severely decreases due to coking of the bio-oil and/or deposition of large molecules or impurities on the catalyst. Furthermore, upgrading catalysts require frequent regeneration or replacement for sufficient conversion.
What is needed in the art is a method for removing contaminants from an fluid stream from a biomass gasification and/or pyrolysis and other plants, which would also improve the amount and quality of the useful material capable of being produced from the fluid stream. The invention, which combines a moving granular bed filter with catalysis, addresses these needs.